Field of the Invention
The invention relates to a system comprising first and second active functional units which are coupled together by means of a synchronization connection in order to supply synchronization signal transitions in handshake from the first to the second functional unit as well as from the second to the first functional unit, each said functional unit having generator circuits for forming signal transitions on the synchronization connection and detector circuits for detecting signal transitions formed on the synchronization connection by the generator circuits of the other functional unit, each said generator circuits being controlled by the detector circuits of the same functional unit so as to respond to signal transitions formed on the synchronization connection by the generator circuits in the other functional unit. A system of this kind is known from U.S. Pat. No. 4,357,658 which is incorporated herein by reference. Notably a two-wire handshake and the interaction thereof with unidirectional and bidirectional data transport is described therein. The handshake utilizes bivalent request and acknowledge signals, each signal transition by the first active functional unit enabling a subsequent signal transition in the second functional unit and vice versa. This handshake will be described in detail hereinafter. The synchronization obtained is satisfactory in many cases when adequate restrictions are imposed as regards the properties of the active functional units to be used. However, the definition of the signal transitions may be insufficient; they may for example be subject to so-called jitter: during a given interval, a sequence of transitions then occurs to and fro between the old and the new signal level. Because the handshake in the known system does not have redundance, errors can occur during jitter: during jitter a signal transition may unduly act as a "real" synchronizing signal transition for the other active functional unit. If the other active functional unit reacts comparatively quickly thereto, such a premature reaction may cause an error in the synchronization. In order to avoid these problems, the response of the receiving functional unit or station can be delayed; the amount of the delay must then be determined by the worst case liable to occur in the other station. When use is made of functional units having clock frequencies which differ substantially from one another, the delay may be large and even then the protection will not be absolute; for example if the critical clock frequency becomes still lower. Alternatively the transmitting station may generate a signal transition having a very flat edge. The receiving station must not react thereto before the transmitting station were to expect to receive the next signal transition; this can take place only if it has eventually detected its own signal transition. In such circumstances a waiting time which is chosen to allow for the worst case must again be incorporated in the receiving station.
The situation can be improved by increasing the number of connection wires used for the synchronization to four (two in both directions). After detection of a received signal transition, each of the active functional units itself then generates a signal transition, that is to say alternately on the one and on the other outgoing synchronization wire. On the one hand, the restrictions as regards the waiting times to be implemented (no longer necessary) or the permissible clock frequencies are thus cancelled. On the other hand, the larger number of connection wires has a severe cost-increasing effect.